The present invention relates to braking controls for electric traction motors and, more particularly, to a braking control system for blending of dynamic and regenerative electrical braking of an inverter powered alternating current traction motor.
Traction vehicles such as locomotives or transit cars which are powered by electric traction motors generally depend upon electrical braking by the traction motors to assist mechanical or friction brakes in stopping the vehicle. In order to provide this electrical braking effort, the traction motors are electrically controlled to operate as electrical generators driven by the rolling wheels of the vehicle. In operating as generators, the traction motors are effective to convert the kinetic energy of the vehicle to electrical energy. The chosen method of disposing of this electrical energy classifies the type of electrical braking being utilized. In general only two types of electrical braking are in common use: dynamic braking in which the electrical energy is converted to thermal energy in resistive loads; and regenerative braking in which the electrical energy is transferred back to the power source.
It is obvious that regenerative braking is a preferred method to use if the power source is capable of accepting the "regenerated" energy and using it for other loads or storing it for later use. However, there are many instances in which a power source is not receptive or not available to accept this energy. In those instances the energy is necessarily dissipated in resistive loads by dynamic braking techniques. It will be apparent then that the use of either dynamic or regenerative braking will generally be dictated by the availability of a power source to accept the regenerated energy. When this availability is not a characteristic of the vehicle but is a variable depending upon certain operating conditions of the vehicle, both dynamic and regenerative braking ability may be incorporated into the vehicle control system with appropriate sensing apparatus for determining when to use dynamic braking and when to use regenerative braking.
Many vehicle control systems are designed for use where the receptivity of the power source itself is variable. For example, the power source may be external to the vehicle such as in electric locomotive or transit car applications in which power is obtained from an overhead catenary or third rail system. In these systems the power source is typically a substation supplying rectified direct current (d-c) power through a plurality of unidirectional conducting means. Accordingly, power cannot be regenerated backward through the unidirectional conducting means to the ultimate source; however, power may be regenerated into the wayside power distribution system for use by other vehicles which are drawing power from the system. The "source" receptivity will, therefore, depend upon whether other vehicles are drawing power from the source and the degree of receptivity will depend upon the number of vehicles drawing power and whether other vehicles are operating in a regenerative braking mode. This problem of power source receptivity has given rise to systems for effecting mixed regenerative and dynamic braking, the mixing process being commonly referred to as "blending."
A typical electrical brake blending system is shown in U.S. Pat. No. 3,657,625 in which a d-c traction motor and associated power system is connected to a d-c power source. For electrical braking the power system includes a dynamic brake resistor and series thyristor connected in parallel with the series combination of the motor armature and a stabilizing resistor. The motor system is connected to the d-c source by means of a diode and a series connected inductor with a capacitor connected between a return line of the power source and a junction intermediate the diode and inductor. During regenerative braking the capacitor voltage will rise if the power source is not receptive. Accordingly, a control system is connected to control the conduction time of the dynamic brake thyristor as a function of the voltage on the capacitor.
A modification of the above-described blending circuit is illustrated in U.S. Pat. No. 3,930,191 in which a pair of dynamic brake resistors and serially connected thyristors are coupled in parallel with the motor armature. This arrangement allows the use of a smaller value for the series stabilizing resistor (since the effective dynamic brake resistor can be varied over a wider range) and increases the efficiency of the system during regenerative braking since a smaller amount of energy will be dissipated as thermal energy in the smaller series connected stabilizing resistor.
In both of the above-described brake blending systems, the dynamic brake resistor-thyristor combination is connected in parallel with the d-c power system during braking. The power system for a d-c traction motor is typically a "chopper." The chopper is essentially a controlled switch which meters motor current by periodically opening and closing. The average value of motor current is thus regulated by varying the ratio of the open-time of the switch to the closed-time of the switch. In present day "solid-state" systems, the chopper includes a power thyristor in the motor current path and a commutation circuit connected in parallel with the power thyristor for applying a reverse voltage to effect turn-off of the power thyristor. Since the dynamic brake thyristor is connected essentially in parallel circuit arrangement with the chopper, it is apparent that the commutation circuit for the chopper can also be used to effect turn-off of the dynamic brake thyristor. Such a system is shown in U.S. Pat. No. 3,593,089. Because commutation circuits capable of operating at typical motor currents and voltages are expensive, this dual use of the chopper commutation circuit makes the use of a dynamic brake thyristor economical.
Persons skilled in the traction vehicle propulsion art are giving increasing attention to replacing d-c traction motors with lighter weight, more maintenance free alternating current (a-c) adjustable speed traction motors, preferably of the 3-phase induction motor type. Such an a-c motor is driven by a power system which can include a polyphase inverter or three single-phase inverters, one connected to each phase of the stator windings of the motor, for supplying 3-phase variable frequency a-c excitation. As is well known, an inverter comprises apparatus for converting d-c to a-c power and generally takes the form of a plurality of controlled switching devices arranged and controlled in a manner to cause current flowing in a load to periodically reverse directions. A detailed description of a plurality of inverters connected for supplying a-c power to a 3-phase a-c induction motor is given in applicant's U.S. Pat. No. 3,890,551 issued June 17, 1975, and assigned to the General Electric Company. As will be apparent from that patent, each phase of the inverter utilizes a pair of main current carrying controlled switching devices, typically thristors, serially connected between relatively positive and negative a-c source terminals, and an a-c load terminal is disposed between the thyristors. By gating on a first of these thyristors, voltage having the same polarity as one of the d-c terminals is applied to a load connected to the a-c terminal. By turning off the first thyristor and gating on the second, voltage of the other polarity is applied to the load. Thus, the inverter alternates the polarity of the potential on the a-c trminal, and the unipolarity source voltage is converted to an alternating load voltage.
As illustrated in the aforementioned U.S. Pat. No. 3,890,551, the main thyristors are paralleled by inversely poled diodes and are periodically turned off by the action of a commutation circuit including a capacitor-inductor ringing or oscillatory circuit which is controlled by an additional thyristor pair. A detailed description of such a commutation circuit is given in U.S. Pat. No. 3,207,974 -- McMurray issued Sept. 21, 1965 and assigned to the General Electric Company.
As explained in the McMurray patent, for the commutating circuit to divert load current from the main thyristor, it must be capable of supplying a current equal to or somewhat greater than the actual load current through the main current carrying thyristor. Since the commutating current is provided from a capacitor, the current magnitude is a function of the initial voltage on the capacitor. Thus, if motor current increases while source voltage decreases, one of the main thyristors may fail to be commutated and a shoot-through or simultaneous conduction of both main thyristors can occur. A shoot-through condition places an undesirable short circuit on the d-c source. The possibility of commutation failure can be minimized by including protection circuits which maintain predetermined current-voltage relationships in the inverter. Such a protection circuit is illustrated in applicant's U.S. Pat. No. 3,859,579 issued Jan. 7, 1975, and assigned to the General Electric Company.
To implement the blending of regenerative and dynamic braking in an inverter powered 3-phase a-c induction motor system, a resistor-thyristor combination could be connected to each phase of the inverter a-c terminals. This would tend to minimize ripple currents created during dynamic braking. As an alternative to the use of three resistor-thyristor combinations connected on the a-c side of the inverter, a single resistor-thyristor braking circuit could be connected between the d-c terminals of the inverter. For this arrangement a series isolating resistor is preferably connected between the inverter and the braking circuit so that the voltage across the d-c terminals of the inverter can rise to a level sufficiently high to ensure successful commutation of the relatively large braking currents in the main thyristors. (The braking effort produced by the motor is directly related to the generated current.) The use of such an isolating resistor in an inverter powered system is described and claimed in the aforementioned U.S. Pat. No. 3,890,551 in which a dynamic brake resistor and switch are connected between the power source terminals. However, that patent does not disclose blending of regenerative and dynamic braking. Because of the isolating resistor, a dynamic brake thyristor could not be effectively commutated by the inverter commutation circuits and thus for "continuous" control of conduction time requires the use of a separate commutation circuit in the dynamic brake circuit. Since the dynamic brake circuit is connected across the d-c power source terminals, the separate commutation circuit for the dynamic brake thyristor would need to have sufficient capability to operate at source voltage and at a current level equivalent to the maximum rated motor current; however, the use of a commutation circuit of such capability in a braking circuit is economically unattractive.